Core wound paper products are in constant use in daily life. Particularly, bath tissue and paper towels have become a staple in home and industry. Such products usually comprise a roll of a paper product spirally wrapped around a hollow core.
The hollow cores are typically made on a coremaking line and comprise inner and outer plies of linerboard superimposed in face-to-face relationship. Each ply of the linerboard is supplied to a coremaking mandril from a spool of raw material. When the two plies are fed to the coremaking mandril, they are typically helically wrapped in the same direction. During wrapping, the plies are adhered throughout to maintain the desired cylindrical configuration.
Typically, the two plies are adhered together in face-to-face relationship with a full coverage of adhesive at the interface between the inner and outer plies. Full adhesive coverage is preferred to minimize occurrences of core failures due to adhesive cracking or breaking. The adhesive is conventionally applied to the interface between the plies, particularly the outer face of the inner ply during manufacture.
Typically, the two plies are identical. Each ply is made from linerboard having the same basis weight and thickness. Basis weights typically used for cores used in consumer products, such as bath tissue and paper toweling,, typically range from 26 to 46 pounds per 1,000 square feet, with a 30 or 38 pound basis weight per 1,000 square feet being a common choice.
The two plies provide crush resistance for the cores during manufacture, particularly when the cores are horizontally stacked in a converting bin, prior to being wrapped with the paper product. The cores at the bottom of the converting bin must resist being crushed by the cores above while awaiting processing. If a core does not have sufficient horizontal crush resistance, it will either be crushed, blocking the cores from dumping into the converting line or will jam while in the line. Either occurrence causes the converting line to incur a shutdown to clear the jam. Of course, the crushed cores must be discarded after they are cleared from the jam or from the converting bin--further increasing the downtime and associated expense. Such horizontal crushing forces severely test the resistance of the two plies of the core to diametrically opposed forces which are unintentionally applied.
However, the diametrically opposed forces can be intentionally applied to the core and/or the core wound paper as well. For example, one improvement to core wound paper products is illustrated in commonly assigned U.S. Pat. No. 5,027,582 issued Jul. 2, 1991 to Dearwester which shows a core wound paper product compacted by diametrical compression. The core is flattened and packaged for shipment and sale. At the point of use, the consumer rerounds the core by recompressing in the direction of the diametrical elongation which occurs due to the prior flattening operation.
If the two plies of the core have insufficient strength, rerounding will not properly occur. The core will either invert, a phenomenon which occurs when the two opposing halves of the core do not separate from one another but instead move together in the same direction, or else it will be necessary to insert a finger or spindle into the core to effect rerounding. Either occurrence is a highly undesirable nuisance for the user.
Upon examination of the intentionally or unintentionally applied diametrically compressive forces to the core, it becomes apparent that the two plies serve different purposes. The inner ply becomes tensioned while the outer ply is placed in compression. The tensile and compressive loadings occur within the circumferential plane of the respective plies. If the tensile strength of the inner ply, the compressive strength of the outer ply, or the combination thereof is insufficient to withstand the applied loadings, the core will either crush prematurely or not properly reround if diametrically compressed.
Furthermore, if the plies are not properly joined together in face-to-face relationship, intra-ply creep will occur. Intra-ply creep is the phenomenon of one ply moving relative to the other ply. The movement is not in a rotational sense, but rather occurs on a more localized basis as either ply creeps. Generally, the inner ply functions as an anvil against which the outer ply is joined. The inner ply resists the hoop forces caused by diametrical compression particularly at the vertexes of the compressed core.
Furthermore, despite the continuing efforts to minimize material usage, the present state of the art most frequently utilizes cores having two identical plies of the same basis weight. The lower limit of the basis weight is constrained by the application of forces, including but not limited to the aforementioned diametrical compressive forces, which occur during the life of the core-wound paper product. Generally, it is believed aggregate basis weight of the two plies of a typical current core cannot be significantly further reduced without an undue number of premature core failures occurring. A typical prior art core utilizes two plies, each ply having identical basis weights of about 42 pounds per 1,000 square feet.
This constraint against reducing core ply basis weight is unfortunate. Any reduction in the basis weight of the core ply provides several advantages. For example, as the basis weight of either ply is reduced and the associated material usage decreases, the cost to the consumer of the core-wound paper product decreases. Furthermore, less material is needed in manufacture - conserving precious natural resources. Finally, upon disposal, lower basis weight materials impart less volume to landfills.
To date, there have been no attempts in the art to directly address the different functions (resistance to tension, resistance to compression) of the respective inner and outer plies. Nor have there been any attempts in the art to directly address the problems of intra-ply creep.
One attempt in the art discloses a core having an inner ply consisting of very inexpensive grades of paper which function as filler while the outer ply is a high grade paper, such as good quality Kraft. In this attempt the outer ply is relatively thin to provide a smooth outer finished surface. This attempt suffers from the drawback that two different grades of material must be utilized, doubling logistics and inventory problems. Different materials would have different thermal expansion rates. This changes the balance of forces between the plies following temperature changes, and may lead to premature failure when loaded.
Another attempt in the art utilizes a laminate of paper and plastic frictionally held together. This attempt in the art is said to be stress-releasing and hence does nothing to prevent the intra-ply creep problem. Yet another attempt in the art discloses a three-ply core. In this attempt the inner and outer plies are kraft paper while the central ply is a vapor barrier. The vapor barrier may be a polyethylene sheet or a wax or asphalt impregnated paper, which allows intra-ply creep to occur. Illustrative of these prior art attempts are U.S. Pat. No. 2,751,936 issued Jun. 26, 1956 to Dunlap et al.; U.S. Pat. No. 2,755,821 issued Jul. 24, 1956 to Stahl; and U.S. Pat. No. 5,167,994 issued Dec. 1, 1992 to Paulson.
Accordingly, it is an object of this invention to provide a two-ply core which optimizes the strength and usage of both plies. It is further an object of this invention to reduce the total material costs of this core. Furthermore, it is an object of this invention to economize the usage of cellulosic fibrous materials in a core for corewound paper products.